Duoplasmatron-type ion source including a gas reservoir



Mag =19, 1970 G. G. KELLEY DUOPLASMATRON-TYPE ION SOURCE INCLUDING A GAS RESERVOIR Filed June 26, 1968 18 EMAGNET SUPPLY 19 g: UAPRC Q} 11 I s PLY o-1sov i I I I I 110v AC I In" J; I 2 g I: 1 3 f k p \K' it if! F l 1 6 7 I 4- 11:. Q 12 ACCELERATOR 20 SUPPLY \Q M O-100K.V.

INVENTOR.

George 6. Kelley ATTORNEY.

United States Patent 3,513,351 DUOPLASMATRON-TYPE ION SOURCE INCLUDING A GAS RESERVOIR George G. Kelley, Kingston, Tenn., assignor to the United States of America as represented by the United States Atomic Energy Commission Filed June 26, 1968, Ser. No. 740,208 Int. Cl. H01j 7/24, 27/00 US. Cl. 315108 3 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention was made in the course of, or under, a contract with the US. Atomic Energy Commission.

An understanding of the state of the art of high current duoplasmatron-type ion sources may be obtained by considering the devices disclosed in US. Pat. No. 3,238,414, dated Mar. 1, 1966, to George G. Kelley et al., and in US. Pat. No. 3,340,425, dated Sept. 5, 1967, to George G. Kelley.

When the ion beams from those sources are pulsed by switching the arc voltage, as is often desired in the research related to the injection of ion beams into thermonucleartype devices and similar devices, severe transients are experienced in the ion current. The initial current, for example, may be greater than twice the steady state value. The peak ofthis transient must be limited so as to prevent voltage breakdown in the accelerator tube and excessive heating in the beam duct. This limitation, in turn; may restrict steady state operation to a current value lower than that desired. The present invention was conceived to provide a means for substantially reducing the peak transient currents during startup of such ion sources or during pulsed operation thereof, thus permitting use of. higher steady state currents.

SUMMARY OF THE INVENTION It is the object of the present invention to provide an improved high current ion source that may be operated in a pulsed mode without the attendant large transients. This has: been accomplished in the present invention by substantially increasing the effective volume of the region between the intermediate electrode and the anode of a duoplasmatron-type ion source. It has been determined that, when such a larger effective volume has been provided in an ion source, the peak transient currents are substantially reduced, thus permitting use of higher steady state currents in the operation of the ion source than was heretofore possible. Thus, by efiecting a substantial reduction in the peak transient currents, as is accomplished in the present invention, the problems of voltage breakdown in the accelerator tube and the excessive heating in the beam duct, which are characteristic of prior art ion sources, have been substantially solved by the present invention.

BRI-EFZDESCRIPTION OF THE DRAWING The single figure of the drawing is a cross-sectional view of the improved ion source of the present invention.

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DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in the single figure of the drawing, an intermediate electrode 1 is provided with a tapered end portion with an ion exit aperture in alignment with an ion exit aperture in an anode electrode 4. The intermediate electrode 1 encloses a heated cathode element 15 which is connected to a 0-40 ampere filament supply 17. A cooling jacket 2 encompasses the tapered portion of electrode 1 and a coolant flows to this jacket through a cooling tube 3. The jacket 2 is spaced from the anode electrode 4 by means of an insulator 5 to thus keep the electrode 1 at some prescribed distance from the anode electrode 4. Thus, a definite volume 6 is established between the intermediate electrode 1 and the anode electrode 4.

A grounded accelerating electrode 14 with an entrance aperture in alignment with the exit aperture of anode 4 is provided to serve in its conventional manner. A conventional source magnet 13 is provided and it encompasses the upper portion of the intermediate electrode 1 and is connected to a conventional magnet supply 18. All are supply 19 is connected between the cathode 15 and the anode 4. A 0-l00 kv. accelerator supply is connected between ground and the anode 4. A housing 12, partially shown, encompasses the major components of the ion source and is evacuated in a conventional manner. A cooling tube 16 is provided for cooling the anode electrode 4.

In accordance with the above object of the present invention, the cooling jacket 2 is provided with a passageway 7 which communicates with the volume 6. This passageway 7 is connected through a conduit 8 and a conduit 11 to an external volume or reservoir 9 mounted adjacent to the ion source. The capacity of the reservoir 9, for example, is /2 liter, which is very large compared to the volume 6. Provision is made to bring the ion source gas into the reservoir 9 by means of a supply pipe 10, and then the source gas flows from the reservoir 9 through conduits 11 and 8 and passageway 7 into the arc discharge volume 6. The feed gas to the ion source may be hydrogen, for example. The pressure in volume 6 is substantially stabilized by the addition of the reservoir 9 and by the continuous influx of source gas thereinto.

A separate reservoir is utilized in the present invention to efiect a substantial increase in the arc-pumped volume since the volume 6 within the accelerating region cannot of itself be physically altered conveniently to greatly increase its size.

When beams are pulsed by switching the arc voltage in prior art devices, there is a large transient at turn-on. This transient limits the average value of output current from the prior sources, which is about 55 milliamperes, for example, and the transient peak currents are often as high as 200 ma. The transients limit the output currents in such prior devices because of voltage breakdown in the accelerator tube and heating in the beam duct. On the other hand, in the operation of the present device with the reservoir addition thereto, some transient exists when the electrode potential is applied, but the maximum value of current is only about 1.2 or 1.3 times the steady state value. Accordingly, average steady state output currents of molecular ions of ma. are achieved with the present device without excessive transient peak currents.

It should be understood that the operation of the present device regarding the vacuum requirements and the function of the source solenoid 13 is the same as described in the above-mentioned patents. Also, it should be understood that the beneficial modifications set forth in the above patents such as the use of a plasma expansion cup associated with the anode with a solenoid coil encompassing the expansion cup, the use of electron dump fins,

and/ or the use of beam stabilizing accelerating electrodes may be incorporated into the structure of the present device without departing from the spirit or scope of the present invention, if such is desired.

This invention has been described by way of illustration rather than by way of limitation and it should be apparent that it is equally applicable in fields other than those described.

What is claimed is:

1. In a duoplasmatron-type ion source including a heated cathode, an intermediate electrode enclosing said cathode and defining an exit aperture, an anode having a centrally disposed aperture and being in spaced alignment with said intermediate electrode to define an arc gap region therebetween, an ion source solenoid encompassing the major portion of said intermediate electrode, an ion extractor electrode in alignment with said anode in spaced relation thereto, means for energizing said cathode filament and said solenoid, an arc supply voltage connected between said cathode and anode, and an acceleration voltage connected between said anode and extractor electrode, the improvement comprising an external reservoir mounted adjacent to said ion source, tubing means connecting said reservoir to said are gap region between said 4 intermediate electrode and said anode, and means for feeding a source gas into said reservoir and thus into said are gap region, whereby the pressure in said region is substantially stabilized thereby substantially reducing current transients during pulsed operation of said ion source to'thus effectively permit an increase in the average steady state output current from said ion source.

'2. The ion source set forth in claim 1, wherein the capacity of said reservoir is /2 liter.

3. The ion source set forth in claim 1, wherein said source gas is hydrogen.

References Cited UNITED STATES PATENTS 3,238,414 3/1966 Kelley et a1. 313230 X 3,315,125 4/1967 Frohlich 31 l1l 3,340,425 9/1967 Kelley 313-63 X JAMES W. LAWRENCE, Primary Examiner P. C. DEMEO, Assistant Examiner 7 US. c1. X.R. 315-111; 313-63, 161; 250-413 

